When we see records being broken and unprecedented events such as this, the onus is on those who deny any connection to climate change to prove their case. Global warming has fundamentally altered the background conditions that give rise to all weather. In the strictest sense, all weather is now connected to climate change. Kevin Trenberth

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Wednesday, May 15, 2013

by Jeff Masters and Christopher C. Burt, Wunderground Blog, May 15, 2013Does not compute! That must be what residents of Iowa and the Midwest have have been saying to themselves on Tuesday as a ferocious heat wave unprecedented in intensity for so early in the year sent temperatures soaring as high as 108 °F. Just two weeks ago, the deepest snowfall ever measured during any May of record buried a wide swath from Arkansas to Minnesota, with Iowa breaking its all-time snowfall record for May (13” accumulation at Osage on May 1-3.) And how's this for a definition of "Weather Whiplash": Sioux City, Iowa, had their first-ever snowfall on record in the month of May on May 1st (1.4"), but hit an astonishing 106 °F yesterday. Not only was this their hottest temperature ever measured in the month of May, but only two June days in recorded history have been hotter (June, 10, 1933: 107 °F; and June 21, 1988: 108 °F). On May 12th they registered 29 °F, and thus had a 77-degree rise over 56 hours (from 6 a.m. May 12 to 1:30 p.m May 14.)

Figure 1. Intense heat and strong winds fanned a wildfire in northwestern Wisconsin's Douglas County near Gordon on Tuesday, May 14, 2013. The fire spread to 8,700 acres and forced dozens of residents to leave their homes. Image credit: Kevin Harter/Wisconsin Department of Natural Resources, and courtesy of wisconsinrapidstribune.com.

The hottest temperature of all on Tuesday was 108 °F at Tekamah, Nebraska. This is just 2 degrees short of warmest temperature ever recorded in the state of Nebraska during May: 110 °F at Broken Bow in 1895 (exact day unknown). Numerous all-time early season heat records were set on Tuesday, making the event the most notable May wave in the Midwest since multi-day event in 1934. That heat wave was not preceded by unusually cold weather, which is what makes the May 2013 Midwest heat event truly extraordinary. A few notable cases of "Weather Whiplash" from the May 14, 2013, heat wave:

- Rochester, Minnesota, where 14.5” of snow fell just 10-12 days ago (4th greatest snowstorm for any month on record), saw an all-time early season heat record of 97 °F on May 14th. The previous day, May 13th, it was 32 °F in Rochester -- a 65-degree rise in temperature over the course of 36 hours.

- Omaha, Nebraska, hit 101 °F, the earliest 100 °F on record (old record was 102 °F on May 29, 1934). It was 32 °F in Omaha on May 12th! That tied for the coldest so late in the season with 32 °F on May 13, 1997, and also on May 15, 1983.

- Albert Lea and St. James, Minnesota, hit 102 °F (hottest in the state on Tuesday). Both cities had May snow less than two weeks previously. Tuesday morning, it was 27 °F at Crane Lake and Silver Bay, for a same-day state temperature spread of 75 degrees in Minnesota. This is a relatively common figure for California or Texas, but almost unprecedented for a Midwestern state.

- Minneapolis hit 98 °F, the hottest so early in season (next is 99 °F on May 22, 1925). May record is 106 °F on May 31, 1934 (the only time 100 °F has been measured during May).

- Norfolk, Nebraska, hit 103 °F, the hottest temperature ever measured so early in the year (previous record: 103 °F on May 25, 1967). A daily record low of 29 °F was recorded on May 12th, just two days previous.

Fires in Wisconsin and MinnesotaThe intense heat was accompanied by strong winds, which fanned multiple fires in Minnesota and Wisconsin. One fire, near Gordon, Wisconsin, burned 8,700 acres, making it the largest fire in Wisconsin since the Cottonville fire on May 5, 2005, and the largest fire in Northern Wisconsin since the Oak Lake fire of April 22, 1980 (11,418 acres.) The Gordon fire destroyed 47 structures, including 17 homes, and forced the evacuation of several dozen people. The fire was 90% contained Wednesday morning. At least 25 smaller wildfires were reported in Minnesota, and Governor Mark Dayton signed an emergency order on Tuesday to help the Minnesota Interagency Fire Center fight wildfires around the state.

Why the crazy extremes? Blame the jet streamThe position of the jet stream is a critical controller of weather regimes across the U.S., Europe, and Asia. Precipitation-bearing low pressure system ride along the axis of the jet, and the jet marks the boundary between cold, Canadian air to the north, and warm, subtropical air to the south. On average, the jet flows from west to east, but the jet often departs from average. The jet is continually rippling with U-shaped troughs of low pressure that allow cold air to spill southwards. The troughs are counterbalanced by upside-down-U-shaped ridges of high pressure that bring warm air northwards. When these ridges and troughs grow to unusually large amplitude, record extremes of both cold and heat occur adjacent to each other. Often times, the jet will have multiple extreme loops that result in unusual extremes over large portions of the Northern Hemisphere. That was the case Tuesday in Europe, where an unusually strong ridge of high pressure was present over Western Russia, with a companion strong trough of low pressure over the UK. Moscow hit 29.7 °C (85.5 °F) on Tuesday, and several locations within the city rose to 31 °C (88 °F). According to weather records researcher Maximiliano Herrera, this has never happened in the first half of May before. At the same time, up to 3" of snow fell in the UK on Tuesday, a rare occurrence for mid-May. Could climate change be a factor in the extreme gyrations of the jet stream this year? It could, and I discussed some of the possible connections in my April 2013 post: Unusually cold spring in Europe and the Southeast U.S. due to the Arctic Oscillation.

Figure 2. Monday afternoon's jet stream shows the pronounced ridge (upside-down U-shaped curve to the jet stream) over the midwestern U.S., which led to all-time, record-high temperatures for so early in the year. At the same time, a sharp trough of low pressure (U-shaped dip in the jet stream) was present over the East Coast, which allowed cold air from Canada to spill southwards and set record lows -- for example, 44 °F at Tallahassee, Florida on May 14, 2013.